The present invention relates to a process for producing a lithographic printing plate. More specifically, it relates to a process for producing a lithographic printing plate using an electrophotographic method, which can suppress non-uniform charging to thereby obtain a desirable toner image having low fogging.
A conventional process for producing a lithographic printing plate by an electrophotographic method comprises corona-charging an original plate for lithographic printing comprising a water-resistant support having a layer containing zinc oxide and a binder provided thereon, imagewise exposing, toner developing, fixing and etching.
The above-described water resistant support used is a paper to which water resistant property has been imparted, metal foil, or the composite thereof.
Where a paper is used as the support, in order to impart conductivity to the paper, a so-called conductive agent, such as a coating liquid containing an inorganic electrolyte such as sodium chloride, potassium chloride or calcium chloride, or an organic polymer electrolyte such as quaternary ammonium, is used and a paper is impregnated or coated therewith. In this case, the paper is adjusted so as to have a volume electric resistance of about 1.times.10.sup.9 .OMEGA..multidot.cm.
However, where an original plate for lithographic printing is produced using the paper having been subjected to such a conductivity treatment as a substrate, even if a water resistance treatment has been applied to the paper, due to addition of dampening water during printing, the paper is inevitably partially elongated on a roll during printing, viz., the plate elongation cannot be avoided. Thus, various problems may occur during printing such that wrinkles happen on backedge and resister changes by slipping of printing plate during printing.
As a structure for protecting the paper support from water influence, it has been attempted to use a paper support having, for example, a conductive filler-containing polyethylene layer, laminated thereon i.e., to use a conductive laminate paper, as described in, for example, JP-A-58-57994 and 59-64395 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, such a laminate paper involves the disadvantages that a conductive treatment must be applied to a paper support or a resin film, so that the production cost of the support increases, which may undesirably invite high cost of the entire printing plate.
Further, it has been attempted to use a paper having a metal foil, such as aluminum, zinc or copper, adhered thereon (hereinafter referred to as a "metal foil laminate paper") as described in, for example, JP-B-38-17249, 41-2426 and 41-12432 (The term "JP-B" as used herein means an "examined published Japanese patent publication"). In any case, a paper which is impregnated with the above-described conductive agent is used as a paper to be laminated.
When this metal foil laminate paper is used, a paper must be subjected to a conductive treatment. Further, a metal foil is required to adhere to one side or both sides of the paper. Thus, this attempt has the disadvantage that a production cost is higher than that in the above-described laminate paper.
In this case, it is considered to use a support obtained by forming a conductive layer such as a metal foil on an ordinary base such as a polyester base or a polyethylene laminate paper and further forming a photoconductive layer thereon. However, such a support, although being inexpensive, has a low conductivity as the entire support, so that it cannot be practically used. This point will be explained below.
In a lithographic printing plate by an electro-photographic method, the plate is produced according to a plate making method as shown in FIG. 4 that corona charge is applied to both sides of the original plate. In this drawing, master 1' is charged negatively and positively above and below the photoconductive layer by a negative corona discharge unit 12 and a positive corona discharge unit 19, respectively, prior to entering an exposure part 20. In the exposure part 20, the charged master 1' is exposed to imagewise exposure, so that the charge in the exposed area disappears by the conduction of the photoconductive layer, remaining only in the unexposed area. Thus, a static latent image is formed.
However, in the plate making method having the construction as shown in FIG. 4, if a support has a low conductivity, a discharge phenomenon does not occur well, so that an image deteriorates. It is considered that a conductive layer is directly contacted with a conductor so as to ground, thereby charging. In the case of a lithographic printing plate, however, the plate is not repeatedly used, and a fresh plate is always used. Therefore, it is mechanically impossible that the conductor is directly contacted with the conductive layer interposed between a support and a photoconductive layer.
In the plate making method shown in FIG. 4, exposing light irradiated from a light source is condensed by the lens 18 in the exposure part 20. The condensed exposing light forms an image on the master 1' located in the exposure part 20 between guide rollers 15 and 16, which was supplied from the paper supply part 11 by a carrying means and was subjected to the above-described charging treatment. The master 1' is imagewise exposed form an image. This master 1' which has been exposed to light is carried to the developing and fixing part 17 by a carrying means, where toner is adhered to the unexposed part, followed by development. The developed image is fixed, subjected to a desensitizing treatment, and dried to produce the lithographic printing plate.